1. Field of the Invention
The present invention relates to a process for removing nitrogen oxides and sulfur oxides from gaseous mixtures.
2. Description of Information Disclosures
It is known to use ammonia or ammonia precursors to remove nitrogen oxides and sulfur oxides from gaseous mixtures, such as combustion effluents.
U.S. Pat. No. 4,288,420 discloses a process for removing nitrogen oxides and sulfur oxides from a flue gas by reaction in the presence of a catalyst, with excess ammonia to reduce the nitrogen oxides and form ammonium sulfate which is collected with the coal ash particles. The gas leaving the collector passes to a desulfurization unit. The coal ash particles containing ammonium sulfate are heated to release some of their contained ammonia for recycle to the NO.sub.x removal step.
U.S. Pat. No. 4,400,363 discloses first removing SO.sub.x and thereafter removing NO.sub.x from flue gases by reaction with ammonia. See column 3, line 45 to column 4, line 3. The NO.sub.x removal reaction is catalytic.
U.S. Pat. No. 4,272,497 discloses simultaneously removing NO.sub.x and SO.sub.x from waste gases by reacting the gases with ammonia as shown in FIG. 5. The NO.sub.x removal reaction is catalytic. In catalytic deNO.sub.x reaction processes, such as those of U.S. Pat. Nos. 4,400,363 and 4,272,497, the excess ammonia for the deNO.sub.x reaction could not be usefully employed for the SO.sub.x removal stage.
U.S. Pat. No. 4,051,225 discloses a process in which flue gas is treated, catalytically or non-catalytically, with ammonia to remove NO.sub.x and SO.sub.x to produce nitrogen and ammonium hydrogen sulfate. The molar ratio of ammonia to SO.sub.3 and the steam concentrations are maintained within specified amounts, thereby severely limiting the amount of excess ammonia available for the deNO.sub.x step. Such a process has a high steam requirement which also results in a high water content in the treated flue gas.
U.S. Pat. No. 3,900,554 discloses a method for removing NO from a combustion gas in the presence of oxygen by reaction with ammonia. Although the reaction can be conducted with as little as 0.4 mole NH.sub.3 per mole NO, if it is desired to reduce less than 100% of the NO present in the gas, it is preferred to use a higher mole ratio of ammonia per mole NO and thereby provide an excess amount of ammonia to obtain better results. After the reaction, the excess ammonia must be removed or utilized. See also U.S. Pat. No. 4,507,269.
U.S. Pat. No. 4,029,752 discloses a method for removing sulfur oxides from gaseous mixtures utilizing ammonia to produce elemental sulfur. The gaseous mixture containing sulfur oxides are reacted first with ammonia to form ammonium salts, such as ammonium sulfate or ammonium sulfite, and subsequently the recovered ammonium salts are decomposed at elevated temperatures to form elemental sulfur.
It has now been found that the excess ammonia advantageously employed in the gas denitrogenation process, such as the process described in U.S. Pat. Nos. 3,900,554 and 4,507,269 and remaining in the gaseous effluent of the denitrogenation step, may advantageously be utilized to provide the ammonia required in the gas desulfurization, either for the production of ammonium sulfate or for the production of elemental sulfur, such as the process described in U.S. Pat. No. 4,029,752, and thereby decrease the amount of extraneous ammonia for the desulfurization process by combining the denitrogenation process and the desulfurization process in a specified manner. This integration also permits the use of high excesses of NH.sub.3 in the denitrogenation step, thereby increasing the NO.sub.x reduction without causing the emission of a large amount of NH.sub.3 to the atmosphere.